A bip39 seed is typically made of 12 words (128 bit) or 24 words (256 bit) . Despite you can pick whatever words you want, the last one contains a checksum and can't be picked randomly.
This script show you all the possible candidates of the last word for a bip39 seed made of 12 or 24 words.
It also show the missing bits for each word so that you can pick one of the candidates randomly flipping a coin as many times as the missing bits length is and counting 1 for head and 0 for tail.
Download the project as a zip, open a terminal in the directory and type:
python3 main.py
then follow the script instructions.
The code is very minimal and simple so that can be reviewed. In order to avoid that you have to review the bip39 words list, it is in a separate file and you can just delete that one and download it again from https://raw.githubusercontent.com/bitcoin/bips/master/bip-0039/english.txt and save it as english.txt
Use this project at your own risk and only in a safe environment. I DON'T have any responsability of how you will use it and eventual money loss. Don't use this project for your actual wallet if you don't know what you are doing!!